Electrolytic electrophotography



Nov. 28, 1961 E. G. JoHNsoN ETAL 3,010,883

ELECTROLYTIC ELECTROPHOTOGRAPHY Filed March 30. 1956 H67 KO-3W' 15)/ 5Wra/YEYSS yvide a permanent reproduction.

United l States APatent O 3,010,883 ELECTROLYTIC ELECTROPHOTOGRAPHYEdgar G. Johnson, St. Paul, Minn., and Byron W. Neher,

Hudson, Wis., assignors to Minnesota Mining and Manufacturing Company,St. Paul, Minn., a corporation of Delaware Filed Mar. 30, 1956, Ser. No.575,070 '13 Claims. (Cl. 204-18) This invention relates to the formationof permanent visible reproductions of light-images on light-sensitivesurfaces by methods involving electrolysis at the exposedlight-sensitive surface. The process is direct and extremely rapid. Itis useful in the reproduction of all types of light-images, but isparticularly applicable to the printing of enlargements from microfilm.Electrolysis `may be carried out either simultaneously with, orsubsequent to, exposure of the light-sensitve surface to the desiredlight-image. i

Photosensitve sheet materials having surface layers which becomeelectrically conductive when irradiated with light of the properwave-length are 'Well known. Selenium is a typical surface layermaterial. Cuprous oxide has also been used. These materials are highlycolored and hence do not lend themselves to the direct production ofcopies. However, since they conductivity of the surface varies withincident light, such sheets have been found useful in the transfercopying or reproduction of light-images. For example, powderedcoloredresins are electrostatically adhered to the exposed and differentiallycharged surface in a pattern corresponding to the light-patterninitially applied, and are then transferred to a paper or other surfaceand fused in place to pro- The differential conductivity patternproduced by illumination with the lightimage, and which is responsiblefor the rdifferential charge` pattern in the above process, mayalternatively be used in forming reproductions in absorbent papercontaining suitable electrolytes. These various processes permit reuseof the photosensitive sheet material; but such reuse is limited becauseof the fragile or unstable nature of the surface involved, and is likelyto result in the production of ghost images. The process in generalproduces reverse reproductions of light-images.

The present invention, on the other hand, involves the formation of apermanent visible image directly on the photosensitive surface of astable and rugged, normally white or faintly tinted, sensitive sheetmaterial. The image may be either positive or,negative,.i.e., either thesame as, `or the reverse of, the applied light-image in location oflight and dark areas, and'may be either direct or reverse. The image isformed rapidly and with fine detail and effective contrast, and requiresno subsequent heating, developing, fixing or other analogous operations.Ghost image formation is completely avoided.

These and other advantages are obtained, in accordance with theprinciples of this invention, by activating `with a light-image areceptor sheet having a strongly photoconductive water-resistant zincoxide layer on an electrically conductive backing, and thenelectrolyzing an electrolytic developer solution at the light-exposedand electrically conductive surface areas to form a visible n image onsaid sheet, all as will now be described in terms of the followingillustrative but non-limitative specific examples.y

y Example I A suitable light-sensitive sheet material was firstprepared. A flexible lm of transparent cellulose acetate having athickness of about mils (0.010 inch) was first metallized on onesurface, by vapor deposition in a vacuum, with an extremely thin coatingof aluminum.

smooth white coating was found to be between 0.3 and 0.6 mil inthickness. The sheet material was highly waterresistant. y

Sheet material prepared as just described was suspended in a transparentglass cell containing a solution of 28 grams of copper sulfate in 200ml. of water. A tlat electrode of slightly larger area, in this case acopper plate, was suspended in the solution facing and somewhat removedfrom the coated surface of the sheet material. A light-image was focusedon the uncoated surface of the sheet through the glass wall of the cell,the source of the light being a 1D0-watt bulb and providing an intensityof about 70 foot-lamberts. Exposure was maintained for about 5 seconds.A source of potential was then connected across the copper plate and theconductive aluminum layer of the sensitive sheet, the latter beingconnected to the negative pole, and a current of about 15 milliampereswas passed through the system for about 3 seconds. The sheet waswithdrawn and rinsed, and was found to have a negative reproduction ofvthe light-image on the sensitive coating. Non-illuminated areas of thesensitive coating remained white, while they exposed areas were darkenedby deposition of metallic copper thereon.

Equally effective copywas obtained by exposing the coated sheet to thelight-image under dry conditions, and then promptly immersing the sheetin the electrolytic cell and electrolytically developing'the image inthe manner described. l

Silver nitrate solution was substituted for the copper sulfate toprovide equally .effective image development. Nickelous chloride is alsoeffective, and is improved by the addition of sodium thiosulfate. Aparticularly effective developing solution contains 10% nickelouschloride and 5% sodium thiosulfate.

The ratio of pigment to binder in the light-sensitive coating waseffectively varied over wide ranges. At l2 parts of zinc oxide to one ofresin, as in the specific formula just given, the white areas of theprint are sometimes found to contain dark spots, indicating nonuniformor insufficient resistivity. Excellent prints are obtained at lower'ratios, for example at 8:1 and at 4:1. Somewhat less effective printsare obtained at 3:1 ratios of zinc oxide and Pliolite resin, and ,atl2:1 the lightsensitivity is inadequate and the results are decidedlyinferior. These ratios may be specifically different with other specificoxides and resins but will serve to illustrate a generally desirablerange.

Electrically conductive glass plates have been substituted for thepartially transparent metallized cellulose acetate film as a carrier orbase for the light-sensitive coating. A glass having a surface layerhigh in stannic oxide having a surface resistivity of about600 ohms persquare and a light transmission of at least about has proven useful,although somewhat lower resistivity is preferred.

The sensitive surface of such transparent photosensitive coated platesis effectively exposed to the light-image through the transparent plateand simultaneously electrolytically developed, as described in theforegoing example. These plates may alternatively be first exposed tothe light-image and then, without further irradiation, transferred tothe developing station and `separately developed, the light-memory ofthe zinc oxide coating being sufiicient to maintain the necessaryconductivity at the irradiated areas. The latter procedure is equallyeffective on fully opaque plates such as metal plates coated with thesensitive zinc oxide coating.

Opaque plates have been simultaneously exposed and developed bysubstituting a copper wire frame for the copper plate of Example l andthen exposing the sensitive surface of a coated metal plate to alight-image through the frame while carrying out the electrolysis asbefore. Where the plate area is too large for uniform electrolysis inthis manner, a screen is provided in place of the frame, and the screenis moved steadily during electrolysis so as to avoid producing a visibleshadow pattern on the sensitive sheet.

Direct current is preferred, with the light-exposed sensitive sheetmaterial normally being connected to the negative pole of the source,i.e., forming the cathode of the electrolytic developer cell. Due to thenature of the sensitive zinc oxide coating, however, it is found to befeasible to apply -alternating current and still obtain usefully exactand dense reproductions. The coating appears to have a rectifying effecton such current.

Example 2 In this example the sensitized surface is first exposed toalight-image and a visible reproduction is then developed by applying athin layer of a suitable electrolytic developing agent directly againstthe exposed surface and applying an appropriate voltage across theinterface.

In one modification the electrolyte, e.g., a copper sulfate solution asin Example l, is applied to the exposed sheet by brushing with anordinary paintbrush which is connected to the source of potential. Avisible image is produced Within the short time required to draw theelectrolyte-meistened brush slowly over the surface of the sheet.

A thin uniform layer of copper sulfate solution applied to a copper baror drum which is then drawn or rolled across the exposed surfacelikewise permits adequateelectrolysis to produce a visible image.Replacing the copper sulfate with tartarlc acid produces the sameresult, the copper surface of the bar being dissolved and plated out onthe sensitized surface.

Another modification employs a developer-sheet consisting of aconductive sheet of aluminum or copper foil coated with a moist layer ofone part of gelatin and three parts of glycerine and containing a smallamount of copper sulfate or Silver nitrate. It is rolled out intointimate contact with the entire surface of the exposed light-sensitivesheet, and a potential then impressed across the foil and the conductivesheet. Electrolysis takes place at the light-exposed areas, resulting information of a visible image on the light-sensitive sheet.

-The moist gelatin may also be replaced by a sponge, porous paper, orother absorptive material capable of retaining the electrolyte inquantity sufficient to provide the required developing action. Contactbetween sensitized surface and developing surface may be over the entirearea simultaneously, or over a progressively advancing smaller area asobtained with a gelatin-coated roll.

Example 3 The present example employs a normally solid developermaterial rather than the normally liquid or gelatinous electrolyticdeveloper of the foregoing examples.

Polyethylene glycol melting at about 100 C. (Carbo wax 6000) is combinedwith small amounts of ethylene glycol and nickel chloride and coated inmolten form in a thin layer over the oxide-coated surface of thesensitive sheet `of Example l, hardening to a waxy transparent V solid.The sheet is exposed to a light-image and is then developed by slowlydrawing a heated metal rod over the coated surface, the rod and sheetbeing connected to opposite poles of a source of potential. The coatingmelts and permits electrolysis to proceed, thereby forming a negativereproduction of the light-image. The image is fully visible through thecooled and hardened thin waxy surface layer.

Similar results are obtained with coatings of suitably electrolyzablematerials in other heat-liquiiiable normally solid solvent media such aspolyacrylic acid, carboxymethylcellulose plasticized with glycerine, andgelatin plasticized with glycerine. Solvents which liquify at moderatetemperatures produce most effective development, since heating is foundto reduce the light-memory of the zinc oxide coating; but effectiveprints have been obtained with binders melting as high as C. or somewhathigher.

Example 4 In the above examples, development has been achieved by theelectroplating of a metal from a salt solution onto the exposedlight-sensitive surface. Other reactions are also useful.

A. The exposed surface is made the cathode in a system in which theelectrolyte contains diazonium salts plus coupler materials in acidmedium. Colored images are formed on the zinc oxide coating.

A specific electrolyte consists of a one-tenth molar aqueous solution of:a mixture of equimolar proportions of tartaric acid, phloroglucinol,and the Zinc chloride salt of p-diazo-N-ethyl-N-benzylaniline. Thesolution is applied, by brushing, to the coated surface of a sensitivesheet made as described in Example l which has previously been exposedto a light-image. The sheet is connected to the negative, and the brushto the positive side of a suitable source of potential duringapplication of the solution. A blue-black coloration is produced at thelight-struck areas. There is obtained a negative reproduction of theoriginal light-image.

B. The zinc oxide surface is rst coated with a thin layer of a mixtureof diazotizable amines and coupler materials, and an electrolyte is usedwhich contains sodium nitrite. The sensitized sheet forms the anode. Avspecific coating consists of a one-tenth molar aqueous solution of amixture of equimolar proportions of o-dianisidine and beta-naphthol.Electrolysis of the sodium nitrite at the exposed coated surfaceproduces a dark blue color at the light-struck areas. The sheet has alight blue background color.

C. Colloidal charged particles may be deposited from liquid suspensionunder the inuence of the electric potential to form a visiblereproduction of a light-image. Thus, a 1% suspension of Prussian blue inwater produces a blue deposit on light-struck areas of the Zinc oxidecoated sheet when the latter serves as the anode. Simultaneous exposureand development results in rapid printing of high contrastreproductions, and is preferred where the light-image is not required topenetrate any substantial depth of suspension. Intense exposure in theabsence of the colloidal suspension makes possible subsequentdeyelopment of faint but visible images in the presence of thesuspension.

D. Zinc oxide coatings which are initially strongly colored, e.g., bythe presence of suitable oxidizable or reducible dyes, lare visiblyaltered at light-exposed areas 4by electrolysis in aqueous bleachingsolution. This is an example of the Iformation of a positive image inwhich light-struck areas become light and unexposed areas remain dank.One such system employs a surface coating of methylene blue onthe zincoxide coating, the dye being rendered colorless at the light-struckareas of the sheet by electrolysis in water containing a small amount ofcitric acid or equivalent electrolyte.

The colored `sensitive sheet is prepared by dipping the oxide-coatedsheet of Example l into an aqueous solution of methylene blue dye. Thedye is adsorbed on the surface of the zinc oxide particles. The driedsheet is normally blue in color, converting to white on electrolysis.

yreduces the dye to the colorless state.

While the dye has a tendency toy fade on long aging or on exposure tosunlight, under normal conditions the image produced remains legible forat least six months or longer.

It has also been observed that a positive print made as just described,i.e. by reduction to a colorless condition of light-struck areas of amethylene blue surface coating on a photoconductive' zinc oxide paper,may be converted to a negative print by deliberate re-oxidation of theleuco dye, for example by exposure to `gaseous oxygen. The re-oxidizeddye areas are found to be of a distinctly darker blue than the originalcoated sheet. Presumably the distribution or particle size of theadsorbed dye is `altered during the chemical conversion. The final copyis found to be completely stable except for the tendency rto fade slowlyon exposure to sunlight.

E. Positive images are also formed with sheets carrying dyes which aremore diiiicultly reducible than is methylene blue. Celliton Blue BGFExtra a diazonium dye available from General Aniline and Film Corp., isone example. -In such cases, zinc chloride,v preferably together withsodium bisulte, is added to provide a suitable mechanism for controlledreduction of the dye and thedevelopment of a visible image. Themechanism appears to involve the inital liberation of zinc metal, which,particularly in the presence of the sodium bisulfte, Thus, analogousresults are obtained by first developing a visible image on alight-exposed photoconductive zinc oxide copy-sheet by electrolysis in azinc chloride electrolyte in accordance with the method describedunderExample l, and then treating the surface with a solution of thediazonium dye which is reduced and decolorized at the zinc-plated areasbut retained in colored form at unplated areas of the zinc oxidecoating. The solution preferably contains bisultte in addition to thedye.

Images formed as just described will be seen to be positive images. Theyare much more stable against fading than are the sheets carryingtriphenylrnethane dyes, since the diazonium dye does not re-oxidizeunder atmospheric conditions once it has been reduced to the colorlessfor-m.

In all cases, the dye material m-ust be reducible, under the conditionsprovided, to a visibly different state. It should also be substantivetoward the zinc oxide coating so that it remains strongly aixed thereto.

A further variation involves the combination of visibly reducible dyeand conductive `zinc salt with the transparent fusible solid surfacedeveloper coating of Example i3. A solution of methylene blue in amixture of zinc oxide, Phiolite resin binder and toluene-acetone solventmixture was coated on conductive metallized paper, dried, and over-coaedor surface sized with a thin layer of gelatin and zinc chloride appliedfrom aqueous solution. The sheet was exposed to al giht image anddeveloped by brief contact with 'a heated metal rod, the rod and backingbeing connected to the positive and negative sides respectively of acontrolled source ofelectricity. The blue dye was reduced to thecolorless leuco form at the lightstruck areas. Some of the fused surfacelayer was removed by theheated rod; the remainder hardened on coolingand protected the surface of the sheet. On continued exposure to theair, the leuco dye was re-oxidized, the thus affected `areas then havinga visibly darker blue shade than the surrounding areas of the sheet.

Example accomplished by release of soluble components from the sensitivesurface itself. Thus, a nickel acetate has been incorporatedin the zincoxide suspension, eig., by grinding with the oxide in the bindersolution, or alternatively has been applied as a thin surface layer overthe dried zinc oxide coating. For example, finely powdered nickelacetate is dusted over the still sticky surface of the zinc oxidecoating just before drying is completed; or nickel acetate in aqueoussolution, preferably together with small proportions of hydrophilic orwater-soluble binder such as methyl cellulose or gelatin, is coated as avery thin iilm over the oxide coating and dried in place. The sheet isexposed to the light-image and is then contacted with a moistenedcurrent-carryu'ng. roll which is drawn slowly across the treated surfacewhile an electric current is passed between sheet and roll. A visiblereproduction of the light-image is produced on the treated surface. Thesensitivity of the process is indicated by the observation that usefulimages have been developed by this procedure as well as by thosedescribed in connection with Examples l and 2, using as the electrolytea mixture of only 10% water in alcohol.

The electrolytic development of the visible image by any of theforegoing procedures may obviously be carried out under widely differingconditions as regards time, voltage, and other variables. For practicalpurposes, however, it is desirable. that development be completed withina minimum of time, for example Within not more than about l() seconds.It is also desirable to restrict the operating voltages to those whichcan be easily provided and controlled without elaborate and expensiveequipment and without danger or inconvenience to the operator. Voltagesup to not more than about 5() volts fulfill these requirements. Thephotoconductive layer of the copyingpaper must be sutliciently thick toprovide adequate opacity against the conductive backing; as notedelsewhere, about 0.3-0.6 mil of the mixture used in Example l is highlyeffective, although up to 2 mils has been found useful. It is found byinspection and analysis that a coating of at least about 25X l0-6grum/sq. cm. of nickel or analogous amounts of other materials, isrequired to produce a suitably visible image. The intensity of the lightimage is also a factor which must be considered. Within 'these practicallimitations, it may be shown that the conductivity of the light-struckarea ofthe photoconductive layer just prior to electrolytic developmentmust be of the order of 10-4 to 10-'7 mho per cm. At the same time, inorder to obtain good contrast, the conductivity of areas not activatedby light must be not greater than of the order of 1A() to 3/100 that ofthe light-struck areas.

French process zincy oxides in general, and specifically rFor example, acoating made with a photoconductive mixture of zinc and cadmium suldesby the procedures of Example 1 exhibited a photoconductivity of onlyabout 104 mhoy per cm. and did not yield useful copy under theconditions here listed.

In the accompanying drawing:

FIGURES l and 2 are schematic representations of the apparatus andprocedures described in connection with Example l; and

FIGURES 3-7 are schematic representations of alternative apparatus andprocedure.

'In FIGUREr 1,. light from a lamp 10 passes through a negativetransparency 11 and a lens system 12 to produce a light-image which istransmitted through the transparent tank 13 containing theelectrolyzable solution 14 and is focused on the light-sensitive layer16 coated on the rtransparent Vconductive backing member 17, the twoforming the sensitive sheet material 18. Sheet 18 is elec- 7 tricallyconnected to an electrode 19 through a controlled source of potential,which as indicated symbolically oomprises a switch, battery, variableresistor, and milliammeter. With the light-image focused on the sheet18, closing the switch causes a selective electrolysis to proceed at theilluminated areas of the coating 16, resulting in the formation of avisible reproduction of the lightimage on the sheet 18. Where thesolution is copper sulfate and the sheet IS comprises a zinc oxidecoating on a metallized transparent film, the lighted areas are found tobe darkened by a plating of copper metal, as described in connectionwith Example 1.

In FIGURE 2 an opaque sheet 28 is used as the sensitive receptor. ltconsists of a zinc oxide sensitive coating 26 on an opaque metal plate27. The light image from the lamp 20, transparency 21 and lens 22 isfocused on the oxide coating 26, passing through the transparent wall ofthe container 23, the solution 24, and the frameshaped electrode 29.

In FIGURE 3, the zinc oxide coating 36, previously exposed to the lightimage and supported on a suitable conductive backing 37, is coated witha thin layer of electnolyte 34 by means of a paint-brush 33. Electricalconnections to brush and conductive backing, as indicated, provide forelectrolytic development of a visible image.

FIGURE 4 illustrates a modification in which a sheet 48, comprising acarrier web 47, conductive metallic layer 4S, and stronglyphotoconductive zinc oxide layer 46, and having been previously exposedto a light-image, is contacted with a developer sheet 49 having aconductive backing 50 and an absorptive layer 51 containing aconcentrated solution of electrolyzable developer. Closing of the switchresults in electrolysis and visible change at the light-exposed areas ofthe sensitive layer 46, whereas the unexposed areas remain unchanged.

The developer sheet `49 of FIGURE 4 is made into roll form to providethe developer roll 52 of FIGURE 5. On closing the switch and slowlypassing roll 52 over the exposed sensitized photoconductive surface 53of the conductive copy-sheet 54, electrolysis and visible change isproduced at the light-exposed areas.

In the device indicated in FIGURE 6 a conductive roll 65 having anon-absorptive or only moderately absorptive surface is continuallymoistened with suitable liquid from trough 66, applied in predeterminedquantity through metering rolls 67 and 68. With the switch closed, thevexposed light-sensitive conductive sheet 69 is slowly advanced past theroll 65, as indicated by the arrow, thereby developing a correspondingvisible reproduction on the sensitive surface. Y

The device of FIGURE 7 employs the process and sheet material of Example3. The sensitive sheet J70. comprises a conductive backing 71, astrongly photoconductive zinc oxide layer '72, and a `fusible solidsurface layer 73 containing an electrolyzable developer. The sheet isconnected to a source of controlled potential, the other terminal beingconnected to a metal bar or roll 74 which is internally electricallyheated from a source 75. With the roll 74 at the proper temperature, theswitch is closed.

and the roll slowly advanced across the sheet 70, previously exposed tothe desired light-image. A visible reproduction is obtained.

It will be understood from the foregoing that the sensitive sheet may beexposed to a light-image either during or prior to electrolyticdevelopment. For many purposes the latter is preferred. It is found thatsensitive sheets prepared as described in Example l have suicientlightmemory so that they may be developed in darkness provided exposurehas occurred within not more than a few seconds, or at best a fewminutes, prior to development. The procedures described will be seen tooffer means for almost instantaneous development after exposure, as wellas for simultaneous exposure and development.

As previously indicated, the sensitive coatings may be applied to anyconductive base. AConductive glass, metallized cellulosic Webs, andmetal foil have specifically been noted; but paper containing conductivesalts, paper or lm containing acetylene black, regenerated cellulosefilm plasticized with glycerine or other humectant, and many otheranalogous materials have likewise been found to be sufficientlyconductive to be useful.

The copolymer of styrene and butadiene employed in Example l as a binder'for the light-sensitive zinc oxide is a water-resistant, flexible,adherent, film-forming polymer of highly satisfactory properties. lt islight in color, and does not interfere wtih the light-sensitivity of thepigment. It is readily soluble in low cost solvents, yet the solvent maybe removed without difficulty by forced drying. The polymer isrelatively inexpensive and readily available. Other binders meeting mostor all of these requirements include polystyrene, chlorinated rubber,rub- 'oer hydrochloride, polyvinylidene chloride, nitrocellulose,polyvinyl butyral. On the other hand, polymers which are dissolved orsoftened by water, or which are dark in color, or insoluble incommercial solvents, or reactive with the pigment, are found to beineffective. As typical examples, polyvinyl alcohol, polyacrylic acid,and sodium carboxymethyl cellulose are not acceptable as binders for thelight-sensitive sheet materials of this invention.

As previously noted, the ratio of pigment to binder may vary widely inthese light-sensitive coatings. Mixtures of pigments and mixtures ofbinders may be employed if desired, and various other components may beadded to the pigment-binder coatings; electrolytic developers havepreviously been mentioned in connection with Example 5. Dyestuffs may beadded to alter the spectral sensitivity of the coating. While very thincoatings are preferred, the coatings must be suiiiciently thick to avoidshort circuiting or electrical breakthrough and to provide adequatevisual background. Excessive thickness reduces the conductivity of theexposed coating and is wasteful of material. In general, coatings fromslightly less than one-half mil up to about 2 mils in thickness includethe most useful range.

What is claimed is as follows:

1. A method for producing a visible reproduction of an image patternwhich comprises exposing to said image pattern an integralphotoconductive sheet comprising a highly electrically-conductive metallayer and a photoconductive coating overlying and bonded directly tosaid metal layer comprising an N-type photooonductor in particulate formand an organic water-resistant binder, said photoconductive coatingbonded to said metal layer having a conductivity of at least 10-7mho/cm. on exposure to light, contacting the photoconductive sheet onthe exposed surface thereof With an electrolytically-conductive liquidsolution containing a developer material, and creating a direct currentelectrical potential, and thereby causing a current ow, between saidmetal layer and said electrolytic solution while the exposed surface ofsaid photoconductive sheet is in contact with said electrolytic solutionto deposit a material derived Ifrom said developer material on saidexposed surface which effects a visible color change imagewise on saidexposed surface.

2. The method of claim 1 in which said developer material comprises awater-soluble heavy metal salt.

3. The method of claim l in which said developer material comprises awater-soluble inorganic heavy metal salt.

4. The meth-od of claim 1 in which said developer material comprises awater-soluble organic heavy metal salt.

5. The method of claim 1 in which said developer material comprises adiazonium salt.

6. The method of claim 1 in which said developer material comprises areducible dye.

7. The method of claim l in which said developer material comprises adiazotizable amine.

8. The method of claim 1 in which a gelatin layer 9 containing thedeveloper material is coated on said photoconductive coating.

9. A method for producing a visible reproduction of an image patternwhich comprises exposing to said image pattern an integralph-otoconductive sheet comprising a highly electrically-conductive metallayer and a photoconductive coating overlying and bonded directly tosaid metal layer comprising photoconductive zinc oxide in particulateform and an organic water-resistant binder, contacting thephotoconductive sheet on the exposed surface thereof with anelectrolytically-conductive liquid solution containing a developermaterial, and creating a direct current electrical potential, andthereby causing a current iiow, between said metal layer and saidelectrolytic solution while the exposed surface of said photoconductivesheet is in contact with said electrolytic solution to deposit amaterial derived from said developer ma'- terial on said exposed surfacewhich effects a visible color change imagewise on said exposed surface.f

10. A method for producing a visible reproduction of an image patternwhich comprises projecting said image pattern on an integral opaquephotoconductive sheet com prising a highly electrically-conductiverue-tal layer and a photoconductive coating overlying and bondeddirectly to said metal layer comprising photoconductive zinc oxide inparticulate form and an organic water-resistant binder to form a latentreproduction thereon, subsequently contacting the photoconductive sheeton the exposed surface thereof with an electrolytically-conductiveliquid solution containing a developer material, and creating a directcurrent electrical potential, and thereby causing a current flow,between said metal layer and said electrolytic solution while theexposed surface of said photoconductive sheet is in contact with saidelectrolytic solution to deposit a material. derived from saiddeveloper" material on said exposed surface which effects a visiblecolor change or" said latent image on said exposed surface.

11. A method for producing a visible reproduction of a light image whichcomprises exposing to said light image an integral photoconductive sheetcomprising a highly electrically-conductive metal layer and aphotoconductive coating overlying and bonded directly to said metallayer comprising photoconductive zinc oxide in particulate form and anorganic water-resistant binder, subsequently contacting the just exposedphotoconductive sheet on the exposed surface thereof with a conductiveabsorbent carrier containing an electrolytically-conductive aqueoussolution containing a developer material, and applying a direct currentelectrical potential, and thereby causing a current flow, between saidmetal layer and said conductive carrier while the exposed surface ofsaid photoconductive sheet is in contact with said conductve carrier todeposit a material derived from said developer material on said exposedsurface which effects a visible color change imagewise on said exposedsurface.

12. A method for producing a visible reproduction of a light image whichcomprises exposing to said light image an integral photoconductive sheetcomprising a highly electrically-conductive metal llayer and aphotoconductive coat-ing overlying and bonded directly to said metallayer comprising photoconductive zinc oxide in particulate form, anorganic Water-resistant binder, and

10 an electrolyte and a water-soluble developer material in dry form,moistening the exposed surface of said photoconductive sheet with waterto solubilize at least a portion of the aforesaid electrolyte andWater-soluble developer, and applying a direct current electricalpotential, and thereby causing a current flow, between said metal layerand said moistened surface of said photoconductive sheet to deposit amaterial derived from said developer material on said exposed surfacewhich effects a visible color change imagewise on said exposed surface.

13. The method for producing a visible reproduction of a light imagewhich comprises exposing to said light yimage an integralphotoconductive sheet comprising a highly electrically-conductive metallayer, a photoconductive coating overlying and bonded direct-ly to said`metal layer comprising photoconductive zinc oxide in particulate formand an organic water-resistant binder and an outer surfacelayeroverlying and bonded to said photoconductive coating comprising asolid fusible electrolyte kcontaining a developer material, liquefyingby heating said outer fusible layer of said just exposed photoconductivesheet forming a liquid solution of the electrolyte, and applying adirect current electrical potential, and thereby causing a current flow,between said metal layer and said liquid electrolytic solution todeposit a material derived from said developer material on said exposedsurface which effects a visible color change imagewise on said exposedsurface.

kReferences Cited in the le of this patent UNITED STATES PATENTS 168,465Edison Oct. 5, 1875 1,902,213 Brockway Mar. 21, 1933 2,083,249 ThomsonJune 8, 1937 2,297,691 Carlson Oct. 6, 1942 2,319,765k Talmey May 18,1943 2,433,632 Solomon Dec. 30, 1947 2,541,488 Vanselow et al. Feb. 13,1951 2,633,796 Pethick Apr. 7, 1953 2,692,178 Grandadam Oct. 19, 19542,735,785 Greig Feb. 21, 1956 2,744,859 Rines May 8, 1956 2,758,939Sugarman Aug. 14, 1956 2,798,959 Moncrief-Yeates July 9, 1957 2,798,960Moncrief-Yeates July 9, 1957 2,808,328 Jacob Oct. 1, 1957 2,837,471 LawJune 3, 1958 FOREIGN PATENTS 464,112 Great Britain Apr. 12, 1937 OTHERREFERENCES Graphic Arts Monthly

1. A METHOD FOR PRODUCING A VISIBLE REPRODUCTION OF AN IMAGE PATTERNWHICH COMPRISES EXPOSING TO SAID IMAGE PATTERN AN INTEGRALPHOTOCONDUCTIVE SHEET COMPRISING A HIGHLY ELECTRICALLY-CONDUCTIVE METALLAYER AND A PHOTO CONDUCTIVE COATING OVERLYING AND BONDED DIRECTLY TOSAID METAL LAYER COMPRISING AN N-TYPE PHOTOCONDUCTOR IN PARTICULATE FORMAND AN ORGANIC WATER-RESISTANT BINDER, SAID PHOTOCONDUCTIVE COATINGBONDED TO SAID METAL LAYER HAVING A CONDUCTIVELY OF AT LEAST 10-7MHO/CM. ON EXPOSTURE TO LIGHT, CONTACTING THE PHOTOCONDUCTIVE SHEET ONTHE EXPOSED SURFACE THEREOF WITH A ELECTROLYTICALLY-CONDUCTIVE LIQUIDSOLUTION CONTAINING A DEVELOPER MATERIAL, AND CREATING A DIRECT CURRENTELECTRICAL POTENTIAL, AND THEREBY CAUSING A CURRENT FLOW, BETWEEN SAIDMETAL LAYER AND SAID ELECTROLYTIC SOLUTION WHILE THE EXPOSED SURFACE OFSAID PHOTOCONDUCTIVE SHEET IS IN CONTACT WITH SAID ELECTROLYTIC SOLUTIONTO DEPOSIT A MATERIAL DERIVED FROM SAID DEVELOPER MATERIAL ON SAIDEXPOSED SURFACE WHICH EFFECTS A VISIBLE COLOR CHANGE IMAGEWISE ON SAIDEXPOSED SURFACE.